Tab raised an issue with CSS Variables in that if you want one to be a calc()
expression, but then use it *inside* a calc() expression, it's invalid.
There are two ways around this:
1. Allow calc() inside calc(); define it to be equivalent to bare parentheses.
2. Define bare parentheses as equivalent to (or a replacement of) calc()
when used as a component value.
Comments?
~fantasai